This invention relates to the field of digital image processing, and more specifically to a digital surrogate for a photographic polarization filter.
One of the most useful filters in photography is the polarization filter (“physical polarization filter”). The physical polarization filter comprises a piece of treated glass, gelatin, plastic or other substrate disposed in front of a camera lens which allows only light of a certain polarization to pass through to the recording medium, and blocks, or filters out, light with unwanted polarization. Although being key for the physical polarization filter, it is of no further matter for this patent what the polarization of light is. It will be evident to those skilled in the art that the literature on the physics of light or other electromagnetic waves may be referenced for further information.
The physical polarization filter serves basically three needs: (1) it is able to remove reflections, i.e., you can use this filter to photograph the surface of a lake or through a window, without surface reflections (typically the reflection of the sky) and to see what's underneath the surface or behind the window; (2) it is capable of darkening the sky, which is a wanted effect in many photographs; and (3) it creates a different and enhanced contrast, such as crystal-clear colors and a brighter white. Rotation of a physical polarization filter results in varying degrees of these effects, depending upon the relative position of the sun, or other source of polarized light. When rotated to a rotation angle θ of 90 degrees, the sky darkening effect is at its maximum.
Unfortunately, once a photograph is recorded and it is on film, paper computer, or other recording medium, it is impossible to recover the polarized data out of the image. In other words, it is no longer possible to filter out the light waves of a certain polarization once the recording is taken. It would therefore be very important for digital image editors to have a “digital polarization filter” which simulates most of the effects of a physical polarization filter, since this would enable users to apply a polarization filter after the image was recorded, and to vary the polarization effect to any desired extent. It would also permit digital cameras to add a polarization filter simulation as an image was recorded, even if no physical filter was present.
What is needed is a method to treat the colors of a digital image to simulate the way that the image would look if it had been taken through a physical polarization filter. Such a method, or “digital physical polarization filter,” should be able to remove some surface reflections, darken the sky in a desired way, and make the colors and the contrast appear as if the image was taken through a physical polarization filter.